In the information systems presently used in computers and word processors, data (i.e., programs and files) are stored and retrieved through the disk drive. The disk drive typically has three components: the read-write head ("head"), the actuator arm, and the hard magnetic disk ("disk"). In the conventional disk drive the actuator arm moves the head over the disk so that the circuitry on the head can magnetically transmit bits of information between leads and the disk. FIG. 1 discloses a conventional disk wherein a substrate 5 (typically an Al/Mg alloy) has deposited thereon, consecutively, an electroless or anodized coating (Ni/P) 6, a magnetic coating 7, a protective overcoat 8, and a liquid lubricant 9.
Due to the high density of information typically stored on a disk, the head must come very close to the disk during data transmission in order to insure accurate transfer. Accordingly, the space between the head and the disk (called a "flying height" or "air gap") is often between about 100-150 nm (4-6 microinches). At such extremely small distances, both the head and the disk must be very flat. Thus, the material used for the disk should be very stiff and amenable to a fine finish.
When the disk and actuator arm move relative to each other, an air flow develops and allows the head to "float" above the disk. During operation, the head's ability to float prevents wear-inducing contact between the head and disk which degrades the accuracy of data transfer. However, the starting and stopping of the disk or arm often produces physical contact between the head and disk. Therefore, it is also desirable to make the disk substrate from a material which is wear resistant.
As noted above, most conventional disks are made from a Mg/Al alloy overcoated with Ni/P and a magnetic film. This alloy has been selected as the material of choice for the disk due to its superior wear resistance, stiffness and polishablity, and performs well in the current relatively large disk drives.
However, increasing speed and capacity requirements are forcing disk drives to become smaller and smaller. For example, the current hard disk is about 65-275 mm in diameter and 0.64 mm to 1.5 mm in thickness, and is expected to be less than about 38 mm in diameter and less than about 0.4 mm in thickness in the future. Likewise, the air gap will be reduced to no more than 50 nm (2 microinches). At these dimensions, it is believed that use of Al/Mg alloys as disk substrates will be problematic in that the elastic modulus of Al/Mg alloys (only about 80 GPa) will not provide the stiffness required in the thin disks of the future (wherein the required stiffness will likely be at least about 200 GPa).
Faced with this problem, the art has considered using alternative ceramics. It has been suggested that glass, glass ceramics, amorphous carbon, silicon, titanium, polymers, and stainless steels are the likely candidates for the next generation of disk substrates. See B. Bhusan, "Magnetic Slider/Rigid Disk Substrate Materials and Disk Texturing Techniques-Status and Future Outlook", Advances in Information Storage Systems, Vol. 5, 1993, pp. 175-209. Moreover, Bhusan concludes that, of this group, the most promising are glasses, glass ceramics and amorphous carbon because they possess adequate elastic modulus.
One other material which has been suggested for use as a substrate for future disks is CVD silicon carbide. See Am. Cer. Soc. Bull. Vol. 72, No. 3 (March 1993),p.74. However, CVD silicon carbide suffers from the disadvantages of high cost and directional (columnar) grain structure.
JP 62078716 discloses a zirconia based magnetic disk substrate for use in disk drives. However, the surface roughness (Ra) of this material is reported to be only 0.01 um (100 angstroms). JP 62078715 discloses a zirconia based magnetic disk substrate for use in disk drives. However, the low density of this material would likely yield an even lower surface roughness (Ra).
EPO Patent Application 0 131 895 reports a zirconia based magnetic disk substrate for use in disk drives. However, the best surface roughness (Ra) of any disclosed material is reported to be only 0.003 um (30 angstroms). JP 01112518 discloses a zirconia based magnetic disk substrate for use in disk drives. However, the surface roughness (Ra) of this material is reported to be only 5-8 nm (50-80 angstroms).
Accordingly, there is a need for a disk drive material possessing superior wear resistance, stiffness and polishablity.